1. Field of the Invention
The present invention relates in general to variable valve systems of an internal combustion engine, which vary a lift degree or work angle of engine valves (viz., intake and exhaust valves) in accordance with an operation condition of the engine, and more particularly to the variable valve systems of a type that has an actuating mechanism for actuating a control shaft that constitutes part of a valve lift degree varying mechanism.
2. Description of the Related Art
Japanese Laid-open Patent Application (Tokkai) 2002-155716 shows a variable valve system of an internal combustion engine, which generally comprises a drive shaft that is driven by a crankshaft of the engine, a hollow camshaft that is concentrically disposed about the drive shaft to pivot about the same, a valve lift mechanism that induces an open/close operation of intake valves, a valve lift degree varying mechanism that is incorporated with the valve lift mechanism to vary a lift degree of the intake valves, and an actuating mechanism that actuates the valve lift degree varying mechanism in accordance with an operation condition of the engine. The actuating mechanism comprises an electric motor that is operated in accordance with the operation condition of the engine, and a ball-screw type transmission mechanism that transmits the torque of the electric motor to a control shaft that is a part of the valve lift degree varying mechanism.
The ball-screw type transmission mechanism comprises a housing, a ball-screw shaft that is rotatably held in the housing through bearings and driven by the electric motor, a ball-nut that is meshed with the ball-screw shaft and moved axially along the ball-screw shaft upon rotation of the shaft, and a link mechanism that transmits the motion of the ball-nut to the control shaft while converting the straight line motion to a rotary motion.
The link mechanism comprises a link member that has one end pivotally connected to the ball-nut, and an arm member that has one end pivotally connected to the other end of the link member and the other end connected through a bolt to an end of the control shaft.
Within the housing of the ball-screw type transmission mechanism, there is arranged a potentiometer that detects an angular position of the control shaft. The potentiometer comprises a center pin that is fixed to the arm member, a center arm that is engaged with the center pin to be turned by the same, and a sensor portion that derives the angular position of the control shaft based on an angular position that the arm member assumes.
Under operation of the engine, an information signal from the potentiometer is fed to a control unit together with other information signals from various sensor means, and thus, in accordance with the engine operation condition, the electric motor is operated to run by a controlled degree or time in positive or negative direction. Due to running of the motor, the ball-screw shaft is turned about its axis causing the ball-nut to move axially therealong. The movement of the ball-nut is transmitted to the control shaft through the link member and the arm member thereby to vary or control the rotation direction and rotation degree of the control shaft. With this, the lift degree (or work angle) of the intake valves is continuously varied in accordance with the engine operation condition, so that the engine can exhibit a satisfied engine performance in all operation mode from a low speed condition to a high speed condition.